Allow read replication recovery from last position

litestream.go

@@ -10,6 +10,7 @@ import (
 	"math"
 	"os"
 	"path/filepath"
+	"regexp"
 	"strconv"
 	"strings"
 	"time"
@@ -191,6 +192,49 @@ func (itr *WALSegmentInfoSliceIterator) WALSegment() WALSegmentInfo {
 	return itr.a[0]
 }
 
+type BufferedWALSegmentIterator struct {
+	itr      WALSegmentIterator
+	buffered bool
+}
+
+// NewBufferedWALSegmentIterator returns a new instance of BufferedWALSegmentIterator.
+func NewBufferedWALSegmentIterator(itr WALSegmentIterator) *BufferedWALSegmentIterator {
+	return &BufferedWALSegmentIterator{itr: itr}
+}
+
+// Close closes the underlying iterator.
+func (itr *BufferedWALSegmentIterator) Close() error {
+	return itr.itr.Close()
+}
+
+// Peek returns the next segment without moving the iterator forward.
+func (itr *BufferedWALSegmentIterator) Peek() (info WALSegmentInfo, ok bool) {
+	if !itr.Next() {
+		return WALSegmentInfo{}, false
+	}
+	itr.buffered = true
+	return itr.itr.WALSegment(), true
+}
+
+// Next returns the next segment. If buffer is full, this call is a no-op.
+func (itr *BufferedWALSegmentIterator) Next() bool {
+	if itr.buffered {
+		itr.buffered = false
+		return true
+	}
+	return itr.itr.Next()
+}
+
+// Returns an error that occurred during iteration.
+func (itr *BufferedWALSegmentIterator) Err() error {
+	return itr.itr.Err()
+}
+
+// Returns metadata for the currently positioned WAL segment file.
+func (itr *BufferedWALSegmentIterator) WALSegment() WALSegmentInfo {
+	return itr.itr.WALSegment()
+}
+
 // SnapshotInfo represents file information about a snapshot.
 type SnapshotInfo struct {
 	Generation string
@@ -302,6 +346,32 @@ type Pos struct {
 	Offset     int64  // offset within wal file
 }
 
+// ParsePos parses a position generated by Pos.String().
+func ParsePos(s string) (Pos, error) {
+	a := posRegex.FindStringSubmatch(s)
+	if a == nil {
+		return Pos{}, fmt.Errorf("invalid pos: %q", s)
+	}
+
+	index, err := ParseIndex(a[2])
+	if err != nil {
+		return Pos{}, err
+	}
+
+	offset, err := ParseOffset(a[3])
+	if err != nil {
+		return Pos{}, err
+	}
+
+	return Pos{
+		Generation: a[1],
+		Index:      index,
+		Offset:     offset,
+	}, nil
+}
+
+var posRegex = regexp.MustCompile(`^(\w+)/(\w+):(\w+)$`)
+
 // String returns a string representation.
 func (p Pos) String() string {
 	if p.IsZero() {
@@ -524,7 +594,9 @@ func (hdr *StreamRecordHeader) UnmarshalBinary(data []byte) error {
 
 // StreamClient represents a client for streaming changes to a replica DB.
 type StreamClient interface {
-	Stream(ctx context.Context) (StreamReader, error)
+	// Stream returns a reader which contains and optional snapshot followed
+	// by a series of WAL segments. This stream begins from the given position.
+	Stream(ctx context.Context, pos Pos) (StreamReader, error)
 }
 
 // StreamReader represents a reader that streams snapshot and WAL records.